The invention relates to aluminum surface mount capacitors having anode foil anodized in an aqueous phosphate solution.
Surface mount electrolytic capacitors may be categorized as one of two basic types, those having powder metallurgy bodies, generally fabricated from tantalum or niobium powder, and those having foil coupon anode bodies. The foil coupons used in the latter type of capacitor are usually cut from high purity etched and anodized aluminum foil.
Typically, as shown in FIG. 1, coupons 1 cut from anodized foil are suspended from a bar 2 which provide mechanical registration and electrical connection. The coupons 1 are anodized to form anodic oxide on the cut edges of the coupons prior to coating with a conductive polymer/graphite/silver or other conductive paint and assembly into a finished capacitor complete with a molded or conformal insulating plastic case. The devices are then xe2x80x9cagedxe2x80x9d with voltage applied (to produce devices having stable electrical properties), tested and placed into reels for shipment to customers. Conductive polymer cathodes are preferred due to the relatively high resistivity and low thermal stability of traditional liquid electrolytes used to fabricate most leaded aluminum electrolytic capacitors. Conductive polymer cathode material is desirable over pyrolytic manganese dioxide (used in solid, surface mount tantalum capacitors) due to the degradation of the anodic oxide film which occurs when anodized aluminum is exposed to the temperatures and highly acidic conditions associated with pyrolytic manganese dioxide production from manganese nitrate solutions.
It was discovered that the production of stable surface mount, stacked-foil aluminum electrolytic capacitors having conductive polymer cathode material at high yields is rendered difficult by the instability of the aluminum oxide dielectric towards hydration. This instability can lead to problems in welding the anodes of the capacitors during assembly, elevated initial leakage, and leakage instability on storage in a humid environment.
Aqueous solutions of dicarboxylic salts, such as ammonium adipate, are used to produce the dielectric oxide on the vast majority of aluminum capacitor foil in use today. This oxide has excellent dielectric and capacitance properties, but it is highly susceptible to hydration. The anodized foil is usually coated with a thin phosphate layer by dipping in dilute phosphoric acid and heating the foil to dry the surface, etc. to help resist this hydration upon exposure to humidity, but the thin layer nature of this phosphate coating provides only limited resistance to hydration. The surface phosphate coating is sufficient to protect the foil during handling. For wet aluminum capacitors, where the foil is sealed in a can in contact with a substantially nonaqueous electrolyte, the surface hydration-resistant layer is also sufficient to keep the foil from hydrating during the life of the capacitor. Phosphate salts are also added to the liquid electrolyte to retard hydration. However, for solid electrolytic capacitors with aluminum oxide dielectrics and conductive polymer cathodes this surface hydration-resistant layer provides insufficient protection.
The solid aluminum electrolytic capacitors with conductive polymer cathodes are susceptible to hydration at several points in the manufacturing process. One such point is during the polymerization process. The catalyst/oxidizing agents/doping acids present during in situ production of the conductive polymer cathode layer have been found to be very destructive to the thin hydration-resistant layer on the anodic oxide surface. For example, U.S. Pat. No. 4,910,645 to Jonas describes the application of various polythiophenes to anodized aluminum substrates. In a preferred embodiment of Jonas, 3,4-ethylenedioxythiophene is applied using an iron III salt or an alkali metal or ammonium persulfate to oxidize the 3,4-ethylenedioxythiophene monomer. With either type of oxidizing agent, iron III salt or persulfate, the pH is reduced over the course of the polymerization reaction due to the liberation of acid; in the case of the persulfate salt, sulfuric acid is liberated. It is well known that sulfuric acid tends to have a corrosive effect upon aluminum depending upon the solution temperature and concentration, as well as upon the time of exposure.
Tests have demonstrated that the amount of sulfuric acid generated during the polymerization process is such that over xc2xe of the phosphate coating present on commercially available capacitor anode foil may be dissolved from the surface of the surface of the foil during the conductive polymer application. After polymerization, the capacitors are washed in elevated temperature water ( greater than 50xc2x0 C.) to remove polymerization by products. Because the surface hydration-resistant layer has been damaged, the capacitors are very susceptible to hydration at this point in the process. Hydration during the washing step can lead to inability to weld the capacitor to the lead frame and elevated leakage current, and, therefore, lower yield and quality.
After the process of assembly and molding during which the polymer/carbon/silver paint-coated anode coupons are cut from the support bars, stacked on a lead frame with the polymer-coated ends attached to the lead frame cathode via conductive adhesive and the uncoated ends welded to the lead frame anode portion via resistance or laser welding, the capacitor assemblies are then encapsulated by transfer molding, etc., to produce the finished capacitor. Unfortunately the assembly and molding or other insulating coating application process gives rise to numerous cracks in the dielectric. FIG. 2 shows an aluminum substrate 3 having an aluminum oxide coating 4, a phosphate outer layer of aluminum oxide 5 and a crack 6 in the aluminum oxide. The crack 6 acts as an electrical leakage site when the devices are electrified.
In order to reduce the leakage current, the molded capacitors are electrified prior to testing. Co-pending application, U.S. Ser. No. 09/812,896, hereby incorporated by reference in its entirety,. discloses that aging of aluminum capacitors containing conductive polymer cathodes are enhanced when the capacitors are moist and at an elevated temperature. The moisture contained within the molded devices appears to undergo electrolysis, providing oxygen to the cracks in the anodic oxide, producing a xe2x80x9cplugged crackxe2x80x9d 7 with fresh anodic oxide (FIG. 3). This results in reduced leakage current and oxidative degradation of the conductive polymer adjacent to cracks in the oxide. However, if the phosphate coating on the anodic oxide has been partially or wholly dissolved by the action of acids evolved during the polymerization process, the anodic oxide tends to undergo hydration during moisture exposure prior to electrifying or xe2x80x9cagingxe2x80x9d. This results in increased leakage current and reduced yields.
Similarly, if the anodic oxide is damaged more than slightly during the assembly and molding processes, the anodic oxide will undergo hydration from moisture seeping into cracks in the oxide and causing a lateral spread of oxide hydration 8 underneath the protective layer of phosphate coating the external anodic oxide surface as shown FIG. 4. Unless it is very carefully adjusted, modem, high-speed assembly equipment provides ample opportunity for damage to the anodic oxide. Once the anodic oxide has become hydrated, it is very difficult to reduce the device leakage current. If the hydration is sufficiently severe, the device capacitance will also be reduced due to the formation of bulky hydrated oxide in the pores of the foil. This can result in capacitor failure during storage or use.
The use of ammonium citrate in combination with ammonium phosphate edge formation electrolyte is disclosed in a co-pending application U.S. Ser. No. 09/874,388, which is hereby incorporated by reference in its entirety, as a way improving the hydration resistance of slit foil for use in solid aluminum capacitors with conductive polymer cathodes. This combination of electrolytes can restore hydration resistance in cracks in the oxide produced prior to polymerization and also form a hydration resistant oxide layer on the edges. However, damage to the outer hydration-resistant layer during polymerization and cracks in the oxide produced during the assembly and molding process occur after the edge formation process is performed. Thus, an aluminum oxide that is hydration resistant throughout the entire oxide volume is needed.
Phosphate is a well known hydration inhibitor for aluminum oxide. It can be used as an additive in solution to inhibit hydration (Vermilyea et al.) or incorporated into the oxide. As discussed above, incorporation of phosphate on the surface of aluminum oxide is well known for inhibiting hydration resistance of foil intended for use in wet aluminum electrolytic capacitors.
As an additive, phosphates are used in the so-called xe2x80x9coperating electrolytesxe2x80x9d of liquid electrolyte solution-containing (xe2x80x9cwetxe2x80x9d) aluminum capacitors (referenced in Alexander M. Georgiev""s 1945 book, entitled: xe2x80x9cThe Electrolytic Capacitor,xe2x80x9d page 41, (Ferris Printing Company, New York )).
To produce an anodic oxide containing phosphate on aluminum, good aqueous phosphate solution anodizing results are generally obtained with solutions having a pH of approximately 5 to 6 and at a temperature of approximately 90xc2x0 C. Even under these conditions, phosphate anodizing tends to result in dissolution of a significant amount of the substrate aluminum. This dissolution tends to form deposits of aluminum phosphate on the walls and bottoms of the anodizing tanks used for the process, as well as on the surface of the foil. Additionally, anodizing in phosphate solutions usually results in anodized foil having about 10-15% lower capacitance for a given anodizing voltage than for the same foil (i.e., foil having the same etch structure) anodized in solutions containing carboxylic acid salts such as ammonium adipate. For these reasons, phosphate anodizing solutions have not been used to anodize aluminum for d.c. aluminum capacitors for decades, although some phosphate anodizing is used to produce anodized foil for a.c. motor-start capacitors, which are filled with a liquid or semi-solid glycol-borate electrolyte, where hydration resistance under reverse polarization is critical to proper device performance.
The sensitivity of standard carboxylic acid salt solution anodized capacitor anode foil to hydration, the corrosion by acids produced during conductive polymer application, and the tendency of mechanically damaged foil to undergo lateral hydration at cracks which undercuts the protective phosphate surface coating, all contribute to yield losses during the production of surface mount aluminum capacitors. Such problems mandate the implementation of careful control of polymer chemical solutions, assembly equipment set-up and speed, and humidity exposure before electrical aging of the finished devices, all of which tend to increase the cost of manufacture.
The invention is directed to aluminum surface mount capacitors comprising one or more anode foil coupons wherein the aluminum foil coupons are initially anodized in an aqueous phosphate solution. The anodic oxide film produced in the aqueous phosphate anodizing solution provides extreme resistance to hydration and attack by corrosive anions.
The aluminum surface mount capacitors can be produced at high yield and having high stability. The stability of the oxide on phosphate-anodized coupons also minimizes the cost of production of the finished capacitors by maximizing yield and minimizing bum-in requirements for the finished devices.
The present invention is directed to an aluminum surface mount capacitor comprising at least one aluminum foil anode having a conductive polymer coating wherein the anode comprises a phosphate-anodized aluminum foil coupon. More particularly, the present invention is directed to anodizing an aluminum anode foil in an aqueous electrolytic solution comprising a phosphate.
The present invention is also directed to a method of preparing an anodized aluminum foil anode comprising immersing an aluminum foil coupon in an aqueous electrolytic solution comprising at least one phosphate and then applying an anodizing voltage to the aqueous electrolytic solution.